Efficient infection of both avian and human cells by influenza A H7N9 viruses is supported by a single nucleotide (a building block of RNA) identified by a study published in Nature Communications this week. Influenza A H7N9 viruses have caused sporadic but severe infections in humans since around 2013. The new finding may improve our understanding of how avian influenza viruses can cause disease in humans.

Birds are natural hosts for many influenza viruses, but only a few strains of these viruses can cause disease in humans. The mechanisms allowing a jump from birds to humans are insufficiently understood and adaptation to human cells often results in decreased viral fitness in avian cells. Honglin Chen and colleagues identify a single nucleotide in the genome of circulating H7N9 viruses that increases replication of the virus in human cells, while also supporting virus replication in avian cells. This nucleotide emerged in early 2000 in H9N2 strains and has since spread in avian influenza viruses. The authors show that this nucleotide is part of an RNA sequence motif that binds the human host cellular machinery that supports virus replication.

Monitoring the spread of this sequence in avian influenza viruses, as part of ongoing surveillance programs, could help to identify viruses that have the potential to cause disease in humans.